My wife's 2001 RX 300 VSC and CEL come on after driving for about 3 hours. After stopping for fuel, they both come on immediately after the engine is started. This has happened 3 times after driving for about 3 hours each time. After resetting the codes, the lights stay off, until the next 3 hour trip. Although I can't be sure, I believe the code number pulled was P0172 - "lean condition, bank 2". I'm sure about the description though.

Any thoughts on why this would occur? Gas cap, maybe?

Thanks!

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How many miles on your RX and have you ever replaced any of the A/F(air/fuel) sensors or 02 sensor?
The A/F sensors are pretty easy to replace in the BANKS especially the one closest to the radiator(Bank2)

I have the same year/model that you have and I often get a CEL/VSC light with similar code(s) for either Bank1/Bank2. Most likely it's the A/F sensor on that particlular Bank. BUT, you may also have a small/mild exhaust leak in that general area causing the CEL to come on. The VSC always comes on with the CEL, IDKY?

P0172 is actually "Fuel Injection System Too Rich-Bank1
P0174 is...System Too Lean-Bank2

In either case, get it checkout

 

Thanks for the reply, CharBaby. My wife's RX300 has about 120K miles on it, and I don't think that the A/F sensor, or any sensor for that matter, has been replaced. We are the second owners, and I have all of the maintenance receipts (mostly dealership) of the original owner. No receipt for any sensors.
The VSC /CEL lights would only come on when starting the car after about a 2 + hr. highway trip. I can reset the lights after the trip, and they never come on again until after another 2 hour + trip (for example - after a fuel stop, or a rest area stop). Does this make any sense to you? I thought maybe a lean condition might be generated by stopping and then starting the engine after a long haul.
Thanks for the tip on the P0174 code.

 

Keep up posted on your results.